The invention relates to an unmanned underwater vehicle and a method for recovering an unmanned underwater vehicle.
Unmanned underwater vehicles may be broadly divided into the subclasses of remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). Whereas remotely operated vehicles are usually controlled by a connecting cable, autonomous underwater vehicles fulfill a mission without being constantly monitored by a human operator. However, unmanned underwater vehicles and in particular autonomous underwater vehicles are cost effective tools for carrying out a variety of tasks in the underwater environment, e.g. pipeline surveys and inspections or military tasks.
AUVs usually provide a slightly positive buoyancy enabling the AUV to appear mechanically at the surface after accomplishing its mission or in case of any malfunctions, e.g. of the power supply of the AUV. However, a slightly negative buoyancy of the vehicle can be provided, which is advantageous in case of dangerous loads.
The recovery is one of the most critical operations of the entire mission of a submersible vehicle since any damage to the valuable AUV has to be avoided. To prevent the AUV from harms caused by recovery means, e.g. a hook of a crane, a common method for recovering the vehicle provides releasing a recovery buoy, e.g. the nose cone of a hull of the AUV, and recovering the buoy and the AUV one after another. However, the recovery buoy is releasably attached to the vehicle and connected to the vehicle by a recovery line. The ejected recovery buoy is recovered from the surface and brought on board the operation platform, e.g. a surface vessel, where the recovery line is attached to a recovery system by the crew involved. After attaching the recovery line to the recovery system, e.g. a crane, the recovery operation continues with lifting the AUV by means of the recovery system and the recovery line. The preceding step of recovering the buoy permits a successive recovery of the vehicle without grappling the floating vehicle.
U.S. Pat. No. 7,814,856 B1 discloses a system and an apparatus for underwater work activity incorporating a manned submersible vessel and a remotely operated vehicle (ROV) for deep sea bottom work. The system includes a power buoy which is supported, if not being in use, on a buoy support attached between the stern areas of each of two hulls of the primary vessel. The surface power buoy provides an upstanding RF antenna for receiving and transmitting radio frequency communication signals between the manned vessel and other boats and ships in the area as well as land based RF transmitters. For retrieval of the ROV the system further includes an ROV launch cage structured to house and support and to protect the ROV therewithin, which is supported at a lifting ring by the ROV's umbilical cord.
JP 62 234794A shows an unmanned submarine tool with a recovery device to collect submerged objects. A male type anchoring metal is inserted into a hole of the submerged object. By moving the recovery device backwards the anchoring metal is detached and further moving backwards breaks up the whole body of the recovery device for separation. After separation of the recovery device, buoys float while drawing a high tension rope, which enables a working boat to collect the submerged object by pulling up the high tension rope.
EP 1 125 838 A1 discloses an apparatus for gripping and moving a surfaced underwater craft comprising a crane mounted on-board a ship and provided with articulated arms and a crane cable. The crane is connected at its free end to a device for gripping the craft to be recovered, wherein said device comprises a gripping unit mounted on floats and provided with propulsion means to allow movement of the device parallel to the surface and towards the craft to be recovered.
US 2008/0029015 A1 discloses a recoverable optical fiber tethered buoy assembly, wherein the buoy provides an antenna for communication purposes
U.S. Pat. No. 5,377,165 discloses a communication system for submarines providing two underwater vehicles lack of them comprising a separable nose cone.
A drawback of the successive recovery of the recovery buoy and the AUV is the possibility of the recovery buoy to float in the near vicinity of the vehicle due to strong currents or wind effects or any other inappropriate weather conditions. Moreover, during the recovery operation the recovery means, e.g. the hook of a crane, can come into a weaving motion due to wind effect or roll or pitch movements of a parent vessel. The weaving recovery means may damage the AUV, if the buoy is floating in the near vicinity of the AUV.
Since an autonomous underwater vehicle loses all contact with the surface after launching, it accomplishes its mission following a program, regularly including a pre-programmed mission time. Thus, autonomous underwater vehicles may return to the surface at times with inappropriate recovery conditions, which were not predictable at the time of launching the vehicle.
However, inappropriate recovery conditions lead to a high risk of damaging the vehicle during an attempt to grapple the buoy, complicating the recovery of the vehicle. Therefore, failure of the recovery attempt or even impossibility of a recovery operation have to be taken into consideration.
In view of the above, it is therefore an object of the present invention to provide an unmanned underwater vehicle and a method for recovering an unmanned underwater vehicle, which provide for safe recovery under most weather conditions.